The present invention relates to carbon foam and compposites, and more particularly to a process for producing them.
The extraordinary mechanical properties of commercial carbon fibers are due to the unique graphitic morphology of the extruded filaments. See Edie, D. D., xe2x80x9cPitch and Mesophase Fibers,xe2x80x9d in Carbon Fibers, Filaments and Composites, Figueiredo (editor), Kluwer Academic Publishers, Boston, pp. 43-72 (1990). Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a pitch precursor can be considered to be an interconnected network of graphitic ligaments or struts, as shown in FIG. 1. As such interconnected networks, they represent a potential alternative as a reinforcement in structural composite materials.
Typical processes for producing carbon foams utilize a blowing technique to produce a foam of the pitch precursor in which the pitch is melted and passed from a high pressure region to a low pressure region. Thermodynamically, this produces a xe2x80x9cFlash,xe2x80x9d thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cNovel Hybrid Composites Based on Carbon Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby, Cellular Solids: Structures and Properties, Pergamon Press, New York (1988), Gibson, L. J., Mat. Sci. and Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36(4), (1976), and Bonzom, A., P. Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Then, the pitch foam must be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and xe2x80x9csettingxe2x80x9d the pitch so it does not melt during carbonization. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor, Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M. Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required. The xe2x80x9csetxe2x80x9d or oxidized pitch is then carbonized in an inert atmosphere to temperatures as high as 1100xc2x0 C., followed by subjection to temperatures as high as 3000xc2x0 C. to produce a graphitic carbon foam.
Other techniques utilize a polymeric precursor, such as phenolic, urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), Cowlard, F. C. and J. C. Lewis, J. of Mat. Sci., 2:507-512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969). High pressure is applied and the sample is heated. At a specified temperature, the pressure is released, thus causing the liquid to foam as volatile compounds are released. The polymeric precursors are cured and then carbonized without a stabilization step. However, these precursors produce a xe2x80x9cglassyxe2x80x9d or vitreous carbon which does not exhibit graphitic structure and, thus, has low thermal conductivity and low stiffness. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992).
In either case, once the foam is formed, it is then bonded in a separate step to the facesheet used in the composite. This can be an expensive step in the utilization of the foam.
The process of this invention overcomes these limitations, by not requiring a xe2x80x9cblowingxe2x80x9d or xe2x80x9cpressure releasexe2x80x9d technique to produce the foam. Furthermore, an oxidation stabilization step is not required, as in other methods used to produce pitch-based carbon foams. This process is less time consuming, and therefore, will be lower in cost and easier to fabricate. Moreover, the foam can be produced with an integrated sheet of high thermal conductivity carbon on the surface of the foam, thereby producing a carbon foam with a smooth sheet on the surface to improve heat transfer.
The present invention further relates to a thermally-conductive foam material derived from carbonaceous precursor, and more particularly to a thermally conductive, pitch-derived carbon foam having high thermal conductivity and heat exchanging properties.
The removal of unwanted heat is a frequently encountered problem. Conventional solutions include cooling fans, ice packs and refrigeration systems. In the latter, a working fluid is compressed (condensed) and pumped into an expansive chamber or pipe system where it evaporates, pulling heat from the atmosphere to satisfy its needed latent heat of vaporization, and thus cooling the surrounding environment. Air blown through the heat exchanger may be cooled and circulated to cool larger volumes such as in domestic and automotive air conditioning systems.
Active cooling (refrigeration) typically requires complex equipment including pumps, valves, compressors, etc. Many refrigeration systems require the use of CFCs (Freon), which is considered hazardous or environmentally unfriendly. An evaporative cooling system with a high thermal conductivity medium would offer a simpler, lower cost alternative. There is a need for portable coolers which are lightweight and inexpensive so as to be deployed in the field or in third world countries.
The thermally conductive carbon foam of this invention overcomes the limitations of the prior art.
In addition, the present invention relates to porous carbon foam filled with phase change materials and encased to form a heat sink product, and more particularly to a process for producing them.
There are currently many applications that require the storage of large quantities of heat for either cooling or heating an object. Typically these applications produce heat so rapidly that normal dissipation through cooling fins, natural convection, or radiation cannot dissipate the heat quickly enough and, thus, the object over heats. To alleviate this problem, a material with a large specific heat capacity, such as a heat sink, is placed in contact with the object as it heats. During the heating process, heat is transferred to the heat sink from the hot object, and as the heat sink""s temperature rises, it xe2x80x9cstoresxe2x80x9d the heat more rapidly than can be dissipated to the environment through convection. Unfortunately, as the temperature of the heat sink rises the heat flux from the hot object decreases, due to a smaller temperature difference between the two objects. Therefore, although this method of energy storage can absorb large quantities of heat in some applications, it is not sufficient for all applications.
Another method of absorbing heat is through a change of phase of the material, rather than a change in temperature. Typically, the phase transformation of a material absorbs two orders of magnitude greater thermal energy than the heat capacity of the material. For example, the vaporization of 1 gram of water at 100xc2x0 C. absorbs 2,439 Joules of energy, whereas changing the temperature of water from 99xc2x0 C. to 100xc2x0 C. only absorbs 4.21 Joules of energy. In other words, raising the temperature of 579 grams of water from 99xc2x0 C. to 100xc2x0 C. absorbs the same amount of heat as evaporating 1 gram of water at 100xc2x0 C. The same trend is found at the melting point of the material. This phenomenon has been utilized in some applications to either absorb or evolve tremendous amounts of energy in situations where heat sinks will not work.
Although a solid block of phase change material has a very large theoretical capacity to absorb heat, the process is not a rapid one because of the difficulties of heat transfer and thus it cannot be utilized in certain applications. However, the utilization of the high thermal conductivity foam will overcome the shortcomings described above. If the high conductivity foam is filled with the phase change material, the process can become very rapid. Because of the extremely high conductivity in the struts of the foam, as heat contacts the surface of the foam, it is rapidly transmitted throughout the foam to a very large surface area of the phase change material. Thus, heat is very quickly distributed throughout the phase change material, allowing it to absorb or emit thermal energy extremely quickly without changing temperature, thus keeping the driving force for heat transfer at its maximum.
Heat sinks have been utilized in the aerospace community to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A material that has a high heat of melting is encased in a graphite or metallic case, typically aluminum, and placed in contact with the object creating the heat. Since most phase change materials have a low thermal conductivity, the rate of heat transfer through the material is limited, but this is offset by the high energy absorbing capability of the phase change. As heat is transmitted through the metallic or graphite case to the phase change material, the phase change material closest to the heat source begins to melt. Since the temperature of the phase change material does not change until all the material melts, the flux from the heat source to the phase change material remains tO relatively constant. However, as the heat continues to melt more phase change material, more liquid is formed. Unfortunately, the liquid has a much lower thermal conductivity, thus hampering heat flow further. In fact, the overall low thermal conductivity of the solid and liquid phase change materials limits the rate of heat absorption and, thus, reduces the efficiency of the system.
The general object of the present invention is to provide carbon foam and a composite from a mesophase or isotropic pitch such as synthetic, petroleum or coal-tar based pitch.
Another object is to provide a carbon foam and a composite from pitch which does not require an oxidative stabilization step.
These and other objectives are accomplished by a method of producing carbon foam wherein an appropriate mold shape is selected and preferably an appropriate mold release agent is applied to walls of the mold. Pitch is introduced to an appropriate level in the mold, and the mold is purged of air such as by applying a vacuum. Alternatively, an inert fluid could be employed. The pitch is heated to a temperature sufficient to coalesce the pitch into a liquid which preferably is of about 50xc2x0 C. to about 100xc2x0 C. above the softening point of the pitch. The vacuum is released and an inert fluid applied at a static pressure up to about 1000 psi. The pitch is heated to a temperature sufficient to cause gases to evolve and foam the pitch. The pitch is further heated to a temperature sufficientto coke the pitch and the pitch is cooled to room temperature with a simultaneous and gradual release of pressure.
In another aspect, the previously described steps are employed in a mold composed of a material such that the molten pitch does not wet.
In yet another aspect, the objectives are accomplished by the carbon foam product produced by the methods disclosed herein including a foam product with a smooth integral facesheet.
In still another aspect a carbon foam composite product is produced by adhering facesheets to a carbon foam produced by the process of this invention.
Another object of the present invention is to provide a thermally conductive carbon foam.
Yet another object is to provide a method of producing a cooling effect utilizing a thermally conductive carbon foam.
Still another object is to provide a heat exchanging device employing a carbon foam core.
These and other objectives are accomplished in one embodiment by a thermally conductive, pitch-derived carbon foam.
In one aspect the foam has an open cell ligament composition.
In another embodiment, the objectives are accomplished by a method of producing a cooling effect wherein a thermally conductive, pitch-derived carbon foam is selected. The foam is contacted with an evaporating liquid, and an evaporation of the evaporating liquid is effected.
In still another embodiment, the objectives are accomplished by a heat exchanging device having a thermally conductive, pitch-derived carbon foam core. A fluid impermeable coating covers a portion of the foam core and exposes a portion. The exposed portion. provides access and egress for an evaporating liquid.
In another aspect, there are upper and lower reservoirs in fluid communication with a core and a pumping device in fluid communication with the upper and lower reservoir adapted to deliver the evaporating liquid from the lower reservoir to the upper reservoir.
In still another aspect, the carbon foam is positioned in separate columns to provide a cold storage container with spacing between the columns.
In yet another aspect, relative motion between the foam and heat transfer fluid is developed in the presence or absence of an evaporative liquid by moving the foam, thereby accelerating evaporation and increasing the cooling effect.
Still another object of the present invention is the production of encased high thermal conductivity porous carbon foam filled with a phase change material wherein tremendous amounts of thermal energy are stored and emitted very rapidly. The porous foam that has been filled with a phase change material (PCM) will conduct heat to the phase change material such that the temperature of the phase change material will remain close to the operating temperature of the device. As heat is added to the surface, from a heat source such as a computer chip, friction due to re-entry through the atmosphere, or radiation such as sunlight, it is transmitted rapidly and uniformly throughout the foam and then to the phase change material. As the material changes phase, it absorbs orders of magnitude more energy than non-PCM material due to transfer of the latent heat of fusion or vaporization. Conversely, the filled foam can be utilized to emit energy rapidly when placed in contact with a cold object.
Non-limiting embodiments disclosed herein are a device to rapidly thaw frozen foods or freeze thawed foods, a design to prevent overheating of satellites or store thermal energy as they experience cyclic heating during orbit, and a design to cool leading edges during hypersonic flight or re-entry from space.